1. Field of Use
This invention relates to a closed loop wire feeding and measuring apparatus and to methods of operating the apparatus to feed accurately-measured predetermined lengths of a wire strand axially along a path.
2. Description of the Prior Art
In the manufacture of wire leads, wire feeding and measuring apparatus is used to feed predetermined lengths of a strand of wire from a wire supply reel axially in a forward direction along a path to a wire processing machine. The wire processing machine may, for example, be a wire cutter or cutter-stripper which then severs wire segments of the predetermined length from the leading end of the wire strand. Prior art apparatus typically comprises a pair of endless flexible rotatable tractor-type feed belts (or feed rolls) which engage opposite sides of the wire strand. The feed belts are intermittently driven forwardly by an electric servo-motor which is controlled by a programmable electric controller. The controller is pre-programmed so as to operate the servo-motor and cause the feed belts to feed a predetermined length of the wire strand, i.e. to move the wire strand a predetermined distance along the path.
Some prior art apparatus include sensing means, typically including some type of encoder, to provide electric signals in someway related to the length of wire fed during each operation of the motor and feed belts.
As a practical matter, slippage sometimes occurs between the feed belts and the wire strand being fed and, as a result, more (overshoot) or less (undershoot) than the predetermined length of wire is actually fed along the path during motor and feed belt operation.
In one type of prior art closed-loop apparatus an electric encoder is connected to be driven directly or indirectly by the servo-motor which drives the feed belts. The encoder provides electric signals to the electronic controller indicative that the motor is in operation and the signals are used to control motor operation. Such a prior art system assumes that wire slippage will not occur and that the desired length of wire is actually fed. But, if and when slippage does occur, the encoder cannot recognize and does not provide a feed-back signal indicative of such slippage and the system cannot take corrective action based on the feed-back signal.
In another type of prior art open-loop apparatus, the encoder can detect whether or not a predetermined length of wire has actually been fed during one cycle of motor operation. However, this encoder provides this information to a human operator so that he can take appropriate corrective action. The apparatus itself does not employ the encoder feed-back signal to initiate and/or carry out corrective action.
In the prior art apparatus hereinbefore described, when slippage occurs, either during set-up of the apparatus or during a production run, the human operator must intervene to take corrective action, as by re-programming the controller or adjusting the apparatus. Meanwhile, wire is consumed and wire segments of improper length must be culled from the production line and disposed of. Needless to say this is wasteful of time, labor and material.
Still another type of prior art apparatus comprises an encoder which is connected to a pair of metering rolls located upstream of the feed belts. The metering rolls (and encoder) can detect whether slippage has occurred. Correction of undershoot is achieved by recommencing operation of the feed belts in the forward direction until the proper length is fed. However, if overshoot occurs, a special problem arises. In particular, when the feed belts and wire strand are driven in reverse, the wire strand (which has little columnular strength) tends to kink as the feed belts force it in reverse between the metering rolls (keeping in mind that the wire supply reel has stopped its forward rotation). Therefore, to overcome this problem the metering rolls are provided with a separate reverse drive means connectable to the feed belt drive which operates to drive the metering rolls in reverse, when necessary, to prevent kinking of the wire strand moving in reverse. As a result, the metering rolls do not respond directly to actual reverse motion of the wire strand and the encoder driven by the metering rolls merely provides a signal indicative of how far the metering rolls (not the wire strand) have moved in reverse in response to operation of their reverse drive means. Any slippage of the wire strand relative to the feed belts or relative to the metering rolls goes undetected and the signal information from the encoder is inaccurate. This is unsatisfactory for precise control of wire feeding and measuring apparatus.